1. Field of the Invention
The invention relates generally to the field of electromagnetic surveying. More specifically, the invention relates to a method and apparatus for reducing induction noise in measurements made with a towed electromagnetic survey system.
2. Background Art
Electromagnetic geophysical surveying of the Earth's subsurface includes “controlled source” and “natural source” electromagnetic surveying. Controlled source electromagnetic surveying includes imparting an electric field or a magnetic field into subsurface Earth formations, such formations being below the sea floor in marine surveys, and measuring electric field amplitude and/or amplitude of magnetic fields by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth's surface, or on or above the sea floor. The electric and/or magnetic fields are induced in response to the electric field and/or magnetic field imparted into the Earth's subsurface, and inferences about the spatial distribution of conductivity of the Earth's subsurface are made from recordings of the induced electric and/or magnetic fields. Natural source (magnetotelluric) electromagnetic surveying typically includes deploying multi-component ocean bottom receiver stations and by taking the ratio of perpendicular field components, it is possible to eliminate the need to know characteristics of the natural source.
Controlled source electromagnetic surveying known in the art includes imparting a substantially continuous, time varying electromagnetic field into the subsurface formations by passing time varying electric current through a transmitter antenna. The alternating current has one or more selected discrete frequencies. Such surveying is known as frequency domain controlled source electromagnetic (f-CSEM) surveying. Another technique for controlled source electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying (t-CSEM). In t-CSEM, electric current passed through a transmitter at the Earth's surface (or near the sea floor), in a manner similar to f-CSEM. The electric current may be direct current (DC). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching techniques are possible. Structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields. For example, U.S. Patent Application Publication No. 2004/232917 and U.S. Pat. No. 6,914,433, issued to Wright et al., describe a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (“MTEM”) measurements on or near the Earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The system and method enable subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.
The above methods for f-CSEM and t-CSEM have been adapted for use in marine environments. Cable-based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. See, for example, U.S. Patent Application Publication No. 2006/0238200 filed by Johnstad. The amplitude of electric field signals detected by electrodes on cables such as described in the Johnstad publication may be on the order of fractions of a nanovolt. Accordingly, a particular consideration in the design and implementation of electromagnetic survey receiver systems is reducing the amount of noise that may be induced in the signals detected by the various sensing elements in the receiver system.
It is desirable to have a marine electromagnetic survey system in which receivers can be towed in the water behind a survey vessel, analogously to a marine seismic survey. By towing the receivers, the efficiency of surveying may be substantially increased over methods known in the art in which receivers are deployed on the water bottom and retrieved after part of the subsurface is surveyed. A particular consideration in designing a towed electromagnetic receiver system is that velocity of parts of the system may result in voltages being induced in the system. Such voltages may be sufficiently large so as to make it impracticable to measure signals related to subsurface electromagnetic phenomena. There is a need for a towed marine electromagnetic receiver system which may have enhanced capability of attenuating induction noise in the signals detected with such system.